Electrolytic polishing of stainless steel



Aug. 19, 1952 J. F. KREML 3 3 ELECTROLYTIC POLISHING OF STAINLESS STEEL Filed Jan. 28, 194? ELECT ROPOLISHING ELECTROLYTE COMPOSITION RANGE 90 80-70 60 so 40 3o 20 IO WATER' PLOTTED FOR STAINLESS STEEL ONE AMP. P.S.|., AVERAGE 90C "30C. TESTS FOR 3 AND 5' MIN.

gYWQ/YVM JOHN F. KREML Patented Aug. 19, 1952 UNITED STATES PATENT OFFICE ELECTROLYTIC POLISHING F STAINLESS STEEL JohnjFQKreml, Baltimore, Md., assignor to Arinco Steel Corporation, a corporation of Ohio ApplicationJanuary 28, 1947, Serial No. 724,912 12 Claims. (01. N l -1405) 1 2 v This invention relates to the polishing of copper, molybdenum, tungsten, vanadium, cumeta1, more especially to a process and an eleclumbium, titanium, sulphur, selenium, phostrolyte for electrolytically polishing the same.- phorus and the'lik'e, for special purposes, and a object of my invention is the provision of remainder which is substantially all iron. a simple, direct and thoroughly practical elec- In certain heretofore known polishing operatrol'ytic polishing process which is highly satistions as applied to stainless steel it has been the factory and reliable for the achievement of customary practice to rely solely upon mechanipolished metal, particularly stainless steel. cal expedients for achieving a polished surface.

Another object is that of providing a process The mechanical techniques, including those of for electrolytically polishing metal; which is rubbing and buffing, however, often introduce readily practiced without need for extensive minute scratches on the metal surface and more preparation and conditioning of the electrolye general unevennesses which affect the surface employed, as in beginning the-polishing operaquality of the resulting polished products. A tions after shutdown and cooling of the polishfurther difficulty arises in applying the necesing solution. sa'ry amount of mechanical work for polishing A further object of my invention is the-prowhere the metal to be polished has relatively invision of a highly efiective electrolytic polishing accessible areas by reason of'sha'pe or contour. solution which remains fluid at low tempera- Then, too, in polishing. as under these conditions tures as during shipment'or after shutdown of it often is found that thEDOliShing equipment metal polishing operations employing the same, suffers a great amount of wear andrequires too and which is readily put into use and gives efli frequent replacement for practicability. cient performance under practical conditions of A more recently developed practice of thesuroperation. face conditioning of stainless steelinvolves elec- Other objects of my invention will in part be trolytic polishing in which the steel is made the obvious and in part pointed outther'ei'nafter. anode of a polishing electrolyte. In general, The invention accordingly consists in the comelectrolytic polishing far surpasses mechanical ination of elements, composition of materials, polishing methodsfrom the point of view of time and inthe several operations steps and the re-'- consumed during the-a'ctualpolishingoperations lation of each of the same to one or more of the and results obtained. Electrolytic polishing enothers as described herein, the. scope of the 'ap- 39 ables the achievement of greater surface brilplic'ation of which is indicated in thefollowing liance and improved beauty, there being evenclaims. ness and uniformity of finish despite intricate ,In the singleview of'the accompanying'draw contours or angularities;which-may have been ing I graphically. illustrate the composition enequntered'for po1ishing, w I,

ranges of my electrolyte. I 7 Since the advent of'electrolytic polishing in conductive to a clearer understanding of the-stainless steel. industrieahowever, a number certain. features of my invention, it may be noted of technical difficulties still remain toreflect at this point that-stainless steel, owing to its upon efficiency and economy of the electrom ny useful properties and an ever increasing chemical processes employed. A number of demand fo P'rdducts i e meta-1 has, hereto- 4 thesedifiiculties are traceable back to the. polishfore, 011 y O c s o sb -fi bjctedto po ing solution and electrolyte which has a substani treatment for such" reasons as to provide-ya tial eifect upon the character and merits of the bright Surface fi a yiw D S process. In certain of the processes, the eleche'steel has a dull graYaDD a G aft trolyte is opento'one or more of the objections polishing a es on a brightness which remains 5 of having low electrical-conductivity, poor throw- W h a Permanence as under conditions of ooring power which renders it diificul-t to polish rosive attack. The steel actually varies in its deep-drawn, angular or other "deep-shaped obcorros'ioneresistancei and other properties a'c-' jects, a'toolow polishing rate asapplied tostaincording. to composition and treatment, but for less steel, and a lowboil-ing point leading to exe Sa e Of definition usually is recognized as cessive evaporation. Many of th solutions have containing from. 0.01 to 0.25%. or substantially an exceedingly high init'al'cos re xtre'mely more carbon, and about.10% to 35%. or more expensive t mai ai chromium, these elements being-presentwith or still other pOj without nickel, and with or without. supplemental; additions ofv manganese; silicon, acobalt, 5

3 shipped separately to the customer to avoid solidification at low temperatures before use. Once the ingredients are mixed in proper proportions, the resulting electrolyte has a tendency to solidify at temperatures below desired operating temperatures, thus making it problematical to shut down production line polishing treatment for any prolonged periods of time as during holidays or over week ends without sufiering considerable expense formaintaining the solution at predetermined high temperatures to prevent solidification. Where the temperature of the bath is lowered a hard, glossy difi'icultly soluble salt of the electrolyte forms. glossy substance is crystalline and builds up in the bottom of the polishin tank and along the sides thereof. After solidification sets in, the bringing of the frozen solution back to operating conditions is a slow, tedious procedure, the time consumed perhaps causing loss of production under most urgent needs. Restoration of the electrolyte by the use of steam or other artificial expedients has been found to be troublesome and expensive. Of further significance, the restored electrolyte frequently requires adjustments for losses in strength and efiiciency caused by the restoration treatment, thus making it all the more desirable to avoid the inconvenience and expenses attendant upon solidification.

An outstanding object of my invention accordingly is the provision of a process for electrolytically polishing stainless steel, employing a polishing solution and conditions of operation which are practical and conducive to the achievement of satisfactorily polished stainless steel surfaces, which process is economical in initial cost and maintenance of the electrolyte solution employed, and which is well suited for shutdown at relatively low temperatures over extended periods of time without substantial solidification of the electrolyte.

Referring now more particularly. to the practice of my invention, I electrolytically treat any of a wide variety of steel articles or products, as for example sheet, strip, plate, bars, wire, or shapes having amore complex surface contour such as tubes, grilles, bowls, trim, tools and instruments, using one or more of the products as the anode in an electrolytic bath comprising as principal constituents substantial, amounts of glycolic acid, sulphuric acid and water, thus achieving a polished surface on the metal. The electrolyte advantageously is an aqueous solution composed of or containing, by weight, at least about 20% up to approximately '7 glycolic acid, at least about 15% up to about 55% sulphuric acid and at least about 5% up to about 30% water, these figures being predicated'upon acids in anhydrous state. The composition range employed is'generally indicated by the solid line ring in the accompanying drawing.

In electrolytically polishing the stainless steel articles and products, I maintain a direct current density in the electrolyte ranging; from about 0.2 to or more amperes per square inch of stainless steel surface under treatment, the optimum usually'being approximately 0.5' to 1.0

ampere per square inch. This I supplement by maintaining the solution at an operating temperature advantageously between about 25 C. and 100 C., and preferably between about 80 C. and 100 C. Under these conditions, I achieve polished surfaces on the stainless steels"inshort This hard, 7

to five minutes, this varying of course with circumstances such as the actual concentration of the polishing bath, the current density, the temperature utilized and the nature of the articles being polished. In general, for a particular solution, the higher operating temperatures permit the lower current densities, while the lower operating temperatures require the higher current densities. Moreover, I find that where the higher current densities are employed, say on the order of 2.0 amperes per square inch, or more, some- 'what broader ranges of solution may be employed, that is, 20% to glycolic acid, 10% to 60% sulphuric acid and 2% to 35% water as generally indicated by the dotted ring in the accompanying drawing.

The electrolyte afforded for my electro-chemical polishing process is conveniently shipped in suitable containers as a solution which is ready for direct use without need for admixing of the several ingredients by the consumer. Where desired, of course, the ingredients may be mixed or replenished at the point of use. The electrolyte has the highly important characteristic of resisting solidification under usual temperature conditions of transit and at temperatures considerably below ordinary room temperatures where the application of heat for the polishing process has been discontinued in favor of resuming operations at a later time. As further characteristics, the electrolyte has high electrical conductivity, good throwing power and a sufficiently'high boiling point to enable high temperature use without excessive losses by evaporation. 7

As illustrative ofthe practice of my invention, I employ a suitable tank for containing the electrolyte in the stainlesssteel polishing operations, this container advantageously being lined with acid-resisting material, as for example, lead, ceramic or glass. The electrolyte, which I pour into the tank, preferably contains by weight about 55% glycolic acid, approximately 30% sulphuric acid, and the remainder substantially all water. This solution I find is extremely valuable for achieving an electrolytically polishedsurface on the steel and, moreover, is capable of resisting solidification at temperatures even lower than about 2". C. i

For polishing, I immerse one or more stainless steel products in the electrolyte, illustratively a low-carbon 18-8 chromium-nickel stainless steel sink strainer possessing; a cold-rolled surface having an area of about twenty-square inches and make the same the anode .iby,.connection with a suitable source ,of direct current electrical supply. Also in the. tank-and imme'rsed in the electrolyte are oneoramorerjcathodes such as of lead, copper or other electrically conductive material connected with thesource of'electrical supply. I dispose the chromium-nickelistainless steel sink strainer between the cathodes, such as between two fiat lead platecathodes, the latter being spaced about two inches'away from opposite sides of the product. The cathode area is not critical'but in order to achieve most satisfactory results I usuallyfindcit .best to have the cathodes of a size commensurate withithe bodily extent of the stainless steel to bexpolished, this for obtaining a more uniform polishing action. Where stainless steel articles'ofirregular shape are under treatment; Iloft'en resort'to the use of cathodes which'conformto theg'eneral config'uration'of the articles to avoid iindue removal eriods of time, say for example, in about three!!! of li the l i m imest I place the cathode above the anode to ensure against the formation of gas pockets which otherwise might reduce the effective current density. v

Before initiating the electro-chemical action for polishing the sink strainer product, I heat up the electrolyte to a temperature of about 90 C. for increased effectiveness. The supply of heat for this purpose conveniently is afforded from a suitable heating coil immersed in the electrolyte or built into the tank wall. Then, with the electrolyte heated and maintained at the temperature just named, I effect the electrolytic polishing treatment using a current, say amperes at 5 volts, which courses from the source of electrical supply to the stainless steel strainer product or anode, thence through the heated electrolyte bath to the cathode and back'to the source of supply. After about 10 minutes time during which an electro-chemical removal of stainleess steel from the anode progresses, I shut off the current and subject the resulting polished strainer product to rinsing as in cold Water in a suitable rinsing tank or the like. In the polished condition the strainer has a very high luster and an even degree of polish. The feature of employing a rinsing tank offers the advantage of recovery of electrolyte dragged-off from the polishing tank by the product. The rinse water often is valuable for addition to the electrolyte in replacement for water losses and drag-off and accordingly I frequently avail upon this practice.

My process further is amenable to electro chemically treating stainless steel objects such as wire which are continuously moved through the glycolic acid electrolyte. As illustrative of the continuous-polishing practice, I subject 0.045 inch diameter 18-8 chromium-nickel stainless steel wire to continuous feeding through an electrolyte containing approximately 50% glycolic acid, 33% sulphuric acid and the remainder substantially all water, the operations being in a polishing tank some 10 feet in length and at a feeding speed of about 5 feet per minute longitudinally through the tank. Six equally spaced copper anodes such as rollers illustratively provide contacts for the 'wire, while for example cathodes made of straight lengths ofcopper bus bar extend on opposite sides of the wire for the length of the tank. In way of further example I employ a current density of some 6.5 amperes per square inch of the immersed wire surface and a solution temperature of about 50 0. As the moving stainless steel wire passes from the polishing tank it is advantageous to rinse the polished surface in water as by directly and continuously feeding the metal through a rinsing tank preferably adapted for recovery of dragoff of the electrolyte.

My electrolytic polishing process is successfully practiced on articles and products in which the stainless steel is of widely varying quality whether of the straight chromium or chromiumnickel grade, and reliably gives uniformly pol-' ished surfaces even where surfaces of intricate contour are encountered. Where the metal under treatment possesses a hammered surface such as is often the case of ornamental trim, hardware, and objects of art including frames, bowls and urns, an even, lustrous polish nevertheless is obtained. Other stainless steel surfaces encountered on grilles, trays, and the like, fabricated by the welding of wire, strip and other converted forms, also are polished successfully. The polished surfaces achieved are substantially 6 free 'of pits, scratches or blemishes, and the welded portions likewise are free of burns, thus resisting corrosion and maintaining their attractiveness over a long period f time. The-treatment in accordance with my inventionis at times useful for such purposes as electrolytically removing thicknesses of metal, in the sense of a machining operation, for achieving desired dimensions of products as well as a smooth finish.

The electrolytic polishing process which I pro- .vide, moreover, is economical and capable of being carried out under operating conditions which are easy to control, or shut down as desired, as during the expeditious mass production of p01- ished stainless steel articles of outstanding surface; quality. The equipment and materials needed in the process are readily available and in actual use call for-little space for the electrolytic treatment.

Thus it will be seen that in this invention there are provided an electro-chemical process and an electrolyte for polishing stainless steels in which the various objects hereinbefore noted together with many thoroughly practical ad-'- vantages are successfully achieved. It will also be seen that the process is easy to set up and maintain using asupply ofelectrolyte constituents which areread-ily; shipped or stored, either separately or in mixedjcondition for direct use.

While my invention has been described as being particularly useful for the. electro-chemical treatment of stainless steel, I also employthe same procedure in conditioningorpolishing any of a wide variety of other alloys or metal, among these being aluminum, Inconel and Nichrome.

It will also be understood that whileI regard "glycolic-acid, sulphuric/acid and water as being highly critical constituents in the electrolyte employed in my process, amounts of other constituents which-do not substantially impair effectiveness of the solution also may be present, if desired. For example, ity is, readily apparent that the beneficial results of my process will be partially retained where the glycolic acid is partially replaced by some other aliphatic carboxylic acid, for example citric acid, or even an inorganic buffer, for example phosphoric acid, as employed in prior electropolishing processes.

As many possible embodiments may be made of my invention and as many changes may be made in the embodiment hereinbefore set forth. it is to be understood that all matter described herein is to be interpreted as illustrative and not as a limitation.

I claim:

1. In the production of an electrolytically polished stainless steel product, the art which comprises, subjecting the metal to anodic treatment in an electrolytic bath essentially consisting of 20% to glycolic acid, 10% to 60% sulphuric acid and 2% to 35% water, all figures being by weight and based upon anhydrous acids.

2. In the production of an electrolytically polished stainless steel product, the art which comprises, subjecting the metal to anodic treatment in an electrolytic bath essentially consisting of, by weight and predicated upon anhydrous acids, about 20% to 80% glycolic acid, about 10% to 60% sulphuric acid and the remainder substantially all water, this amounting to not over about 35%, while maintaining a temperature of the electrolytic bath ranging between about 25 C. and C. and a sufiiciently high current density to electrically remove metal from the surface under treatment and achieve a high polish.

3. In the production of an eelctrolytically polished stainless steel product, the art-which comprises, subjecting the steel to anodictreatment in an electrolytic bath essentially consisting of, by weight and predicatedupon anhydrous acids, about 20% to 80% glycolic acid, approximately to 60% sulphuric acid, and about 2% to 35% water, while maintaining a current density of approximately 0.2 to 10 amperes per square inch of metal surface under treatment and a temperature ofthe electrolytic bath ranging between about 25 C. and 150 C.

4. In the production of an electrolytically polished stainless steel product, the art which comprises, subjecting the steel to treatment in an electrolytic bath by weight and predicated upon anhydrous acids consisting of approximately 55% glycolic acid, about 30% sulphuric acid, and the remainder substantially all water.

5. In the production of an electrolytically polished stainless steel product, the art which comprises, subjecting the metal to anodic treatment in an electrolytic bath by weight and predicated upon anhydrous acids essentially consisting of about 20% to70% glycolic acid, about to 55% sulphuric acid, and about 5% to 30% water, while maintaininga current density of approximately 0.5 to 1 .0 ampere per square inch of the metal surfaceundertreatment and a temperature of the electrolytic bath ranging between about 80 C. and 100 C.

6. In the production of an electrolytically polished stainless steel product, the art which comprises, subjecting the steel to anodic treatment in an electrolytic bath by weight and predi cated upon anhydrous acids composed of approximately 55% glycolic acid, about 30% sulphuric acid and the remainder substantially all water, while maintaining a current density of approximately 0.5 to 1.0 ampere per square inch of the metal surface under treatment and a temperature of the electrolytic bath ranging between about 80 C. and 100 C.

7. In the continuous polishing of stainless steel wire, the art which comprises, continuously feeding the wire through an electrolyte essentially consisting of to 80% glycolic acid, 10% to 60% sulphuric acid, and 2% to 35% water, while maintaining anodic contact to said wire while in said electrolyte, all figures being by weight and based upon anhydrous acids.

8. An electrolyte solution of the character described, essentially consisting of as the principal constituents thereof, 20% to 80% glycolic acid, 10% to sulphuric acid and remainder substantially all water up to 35%, all figures being' by weight and based upon anhydrous acids.

9. An electrolyte solution of the character described, essentially consisting of, by weight and predicated upon anhydrous acids, at least about 20% but not over glycolic acid, at least about 10% but not over 60% sulphuric acid, and at least about 2% but not over 35% water.

10. An electrolyte solution having non-freezing tendencies at low temperatures and essentially consisting of, by weight and predicated upon anhydrous acids, about 20% to 70% glycolic acid and about 15% to 55% sulphuric acid, and about 5% to 30% water.

11. An electrolyte solution having non-freezing tendencies at low temperatures and essentially consisting of, by weight and predicated upon anhydrous acids, about 55% glycolic acid, approximately 30% sulphuric acid, and the remainder substantially all water.

12. The method of anodically polishing stainless steel articles which comprises electrolyzing the article as anode'in an electrolyte of approximately the following composition:

Aqueous sulfuric acid by wt.) 8 to 43% by volume Aqueous glycollic acid (70% by wt.) The balance to achieve a mirror-like polish on the article.

' JOHN F. KREML.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,335,354 Ostrofsky Nov. 30, 1943 Dittmar Aug. 14, 1945 OTHER REFERENCES 

1. IN THE PRODUCTION OF AN ELECTROLYTICALLY POLISHED STAINLESS STEEL PRODUCT, THE ART WHICH COMPRISES, SUBJECTING THE ESSENTIALLY CONSISTMENT IN AN ELECTROLYTIC BATH ESSENTIALLY CONSISTING OF 20% TO 80% GLYCOLIC ACID, 10% TO 60% SULPHURIC ACID AND 2% TO 35% WATER, ALL FIGURES BEING BY WEIGHT AND BASED UPON ANHYDROUS ACIDS. 